Superabsorbent polymers are primarily used as absorbents for biological fluids, water and aqueous solutions. Water absorbent materials such as superabsorbent polymers can be employed in various applications, such as in disposable sanitary products (i.e. diapers, incontinence articles, female hygiene products, and absorbent dressings), household articles, sealing materials, humectants (i.e. agricultural products for soil conditioning), anti-condensation coatings, water-storing materials (agriculture/horticulture), absorbent paper products, surgical absorbents, pet litter, bandages, wound dressings and as chemical absorbents. Furthermore, they can also be employed in applications related to the transportation of fresh food or seafood, as well as in food packaging applications.
Polyacrylates, obtained from the polymerization of monomers such as acrylic acids and acrylamides (non-renewable sources), constitute a major portion of the commercially available superabsorbents (Modern Superabsorbent Polymer Technology, Buchholz F. L. and Graham A. T. Eds., Wiley-VCH, New York, 1998). However, their biodegradability is questionable, especially for high molecular weight polymers. Polyacrylates generally contain small amounts of residual monomeric starting materials (i.e. acrylic acids and acrylamides) possessing both toxic and allergenic potential.
Superabsorbent polysaccharide-based grafted-polymers are obtained via the grafting of an unsaturated monomer (acrylonitrile, acrylic acid, acrylamide) onto starch, or, less frequently, cellulose. The so-obtained polymers, also called “Super Slurper”, have shown a water absorption ranging from 700 to 5300 g/g in deionised water, and up to 140 g/g in a 0.9% saline solution (Riccardo P. O., Water-Absorbent Polymers: A Patent Survey. J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1994, 607-662 and references cited therein). Despite their very high water absorption capability, the grafted polysaccharides, prepared by radical polymerization, are not known to be biodegradable or hypoallergenic, nor are they prepared from renewable sources.
Polyaspartates have also been described to offer good absorbing properties (Ross et al. U.S. Pat. No. 5,612,384). However, polyaspartates appear to possess several drawbacks regarding their low molecular weight. Moreover, polyaspartates are prepared from non-renewable sources which constitutes an additional drawback (Koskan et al. U.S. Pat. No. 5,221,733). Furthermore, these polymers are strongly ionic and are thus subject to performance fluctuations in saline solutions.
Carboxymethylcellulose (CMC), and carboxymethylstarch (CMS) (Modern Superabsorbent Polymer Technology, Buchholz F. L. and Graham A. T. ed., Wiley-VCH, Toronto, 1998, 239-241; Gross et al. U.S. Pat. No. 5,079,354; Arno et al. U.S. Pat. No. 4,117,222; Thornton et al. PCT WO 00/35504; Mindt et al. GB 1576475; Couture et al. CA 2,362,006; Annergren et al. PCT WO 00/21581) constitute other known polysaccharide-based superabsorbents. Cost has always been an issue with these superabsorbents, and they can therefore not be used alone in order to compete with the synthetic polymers. Moreover, these polymers are strongly ionic, as is the case for polyacrylates and polyaspartates, rendering them subject to performance fluctuations in saline solutions. Nonetheless, these products can be used in synergistic formulations, leading to cost effective superabsorbent materials (Bergeron CA 2,426,478; Richman et al. U.S. Pat. No. 4,454,055).
Natural polysaccharide-based superabsorbents constitute a very attractive class of polymers, considering that they can be biodegradable and hypoallergenic, in addition to the fact that they are made from renewable sources such as starch. Polysaccharides have been previously used in an extrusion process for the preparation of non-crosslinked starch-based materials as absorbents for liquids (Huppé et al. CA 2,308,537).
The use of extruders as continuous reactors for processes such as polymerization, polymer modification or compatibilization of polymer blends, involves technologies that are gaining in popularity. These technologies are competing with conventional operations with respect to environmental considerations, efficiency and economic operators. In the case of reactive extrusion, several organic reactions can be conducted in extruders, including polymerization, grafting, copolymer formation, molecular network formation, crosslinking, functionalization and controlled degradation (Reactive Extrusion: Principles and Practice, Xanthos M. Ed., Hanser Publishers, New York, 1992). This technology has been largely applied in the preparation of polysaccharide-based products from renewable sources such as cross-linked starches, and in applications such as food texturing products (Salay E. et al., Starch/Staerke, 1990, 42, 15-17; Nabeshima E. H. et al., Carbohydr. Polym., 2001, 45, 347-353; Narkrugsa W. et al., Starch/Staerke, 1992, 44, 81-90; Chang Y.-H et. al. J. Food Sci., 1992, 57, 203-205; Kim C.-T. et al., Starch/Staerke, 1999, 51, 280-286). However, none of the products cited are absorbent or superabsorbent materials.
Cross-linked starches have been exhaustively studied (Kulicke W. M. et al Starch/Starke, 41, 1989, 140-146; Brine et al. PCT WO 01/19404A1; Ameye et al US App. 2003/0143277; Seib et al. WO 99/64508; Dumoulin et al. PCT WO 98/35992). However, these cross-linked starches (gels) are not absorbent or superabsorbent materials.
Glass-like polysaccharide abrasive grits have been prepared by extrusion processes of native or crosslinked starches (Lane et al. U.S. Pat. No. 5,367,068). An extrusion process for the preparation of a natural gum substitute composed of cross-linked starch, using phosphorous oxychloride, has been disclosed (Hauber et al. PCT WO 97/00620). However, these products are once again not absorbent or superabsorbent materials.
The preparation of absorbent materials consisting of crosslinked starch using trisodium trimetaphosphate in a co-continuous water-oil system has been described (Feil et al. EP 0900807). In order to remove the oil and to recover the starch-phosphate derivative, an organic solvent such as cyclohexane was added, followed by washing with ethanol. The use of oils and organic solvents are important drawbacks of this batch process. They dramatically increase the production cost of this absorbent, while simultaneously complicating the process.
There thus remains a need for new polysaccharide-based absorbent or superabsorbent materials that are non-abrasive, hypoallergenic, and biodegradable, and which can be cost-efficiently produced from renewable natural sources.
The present invention seeks to meet these and other needs.